1. Field of the Invention
The present invention relates to an optical system for an endoscope that has a field angle of no less than 200 degrees.
2. Description of the Related Art
Since endoscopes are intended to, for example, observe an inside of a body cavity, optical systems for endoscopes are demanded to have a small size and a wide angle, and in particular, optical systems for urological endoscopes need to be small in a radial direction.
As examples of optical systems having a field angle of no less than 200 degrees, which can be employed in endoscopes, those described in Japanese Patent Application Laid-Open Publication No. 2007-279632 (Patent Literature 1), Japanese Patent Application Laid-Open Publication No. 2008-058387 (Patent Literature 2), Japanese Patent Application Laid-Open Publication No. 2005-227426 (Patent Literature 3) and Japanese Patent Application Laid-Open Publication No. 2009-276371 (Patent Literature 4) are known.
Among them, the optical systems disclosed in Patent Literatures 1 to 3 each include a negative first lens, a negative second lens, a positive third lens, an aperture diaphragm and a positive lens group in this order from the object side and have a field angle of 200 degrees.
Also, the optical system disclosed in Patent Literature 4 includes a negative first lens, a negative second lens, a filter, an aperture diaphragm and a positive lens group in this order from the object side and has a field angle of 200 degrees.